Conventionally, polyolefin resins such as polypropylene and polyethylene resins have been used as sheets, films and molded products in a wide range of areas because they are thermoplastic resins used commonly, are inexpensive and excellent in moldability, chemical resistance, weather resistance, water resistance and electric property. However, substrates composed of these polyolefin resins (hereinafter, polyolefin substrates) are low polar and crystalline, so known as the poorly adhesive substrate differently from polar substrates such as polyurethane resins, polyamide resins, acrylic resins, polyester resins and metals, and have a shortcoming that adhesion and painting between the same type substrates or between the different type substrates are difficult.
As methods for enhancing the adhesiveness of the polyolefin substrate, the surface treatment of the substrate, for example, making a rough surface by polish, or introduction of a polar group by an oxidant, a corona discharge, a plasma treatment or a flame treatment has been conventionally available. However, these have poor reproducibility for obtaining a certain effect, require works with toxicity and risk, and have the shortcoming that the effect is reduced with time. Thus, they have not become common generally.
Thus, the method of previously coating a substrate surface with a pretreating agent having the adhesiveness has been invented. Such a pretreating agent has various names, and is referred to as, for example, an adhesive agent, a binder or a primer depending on its intended use. As the pretreating agent, for example, the resin having a predetermined property is used. The pretreating agents, e.g., a hot melt system where the resin is melted with heat to make the binder, a solvent system where the resin is dissolved in the solvent and an aqueous system where the resin is emulsified using a surfactant have been provided.
As a specific example of them, the method of increasing affinity to the substrates of polyolefin, polyvinyl chloride, polycarbonate, PET, ABS or nylon to enhance the adhesiveness by the method of using a carboxyl group-containing chlorinated polyolefin resin as a coating composition for the binder and the primer, described in Patent Publication JP 2003-321588(A), is available.
Furthermore, the method of increasing affinity with the polyolefin substrate to enhance the adhesiveness by imparting the polar group to polyolefin by graft modification with unsaturated carboxylic acid and an acrylic derivative and using this as a coating agent, described in International Publication No. 2005/082963 Pamphlet, has been found. These methods enable to paint and adhere onto the polypropylene substrate and various polar substrates, but have no adherent effect required practically on the polyethylene substrate which is more nonpolar and poorly adherent.
Polyethylene is broadly classified into low density polyethylene (LDPE), linear low density polyethylene (LLDPE) and high density polyethylene (HDPE) depending on differences of their production methods and resulting density. Adhesion is difficult in common to these substrates because polyethylene is nonpolar, has low surface energy and is highly crystalline.
Thus, in order to solve this problem, it has been attempted that polyethylene is adhered by an adherent primer composed of: a graft polymerized product of an ethylene-vinyl acetate copolymer and unsaturated dicarboxylic acid or anhydride thereof; lowly chlorinated polypropylene or lowly chlorinated polyethylene; and a solvent, described in Patent Publication JP H01-304164(A). However, its effect is limited to high density polyethylene where the adhesion is relatively easy among the above three classifications of polyethylene, and thermal compression bonding at high temperature is required. Thus, there is a problem that the substrate is deformed.
In the method of using crystalline polyamide described in Patent Publication JP H06-145637(A), the adhesion effect is likewise limited to high density polyethylene; a physical treatment to previously form scratch marks on an adhesion surface by a wire brush is required; and further, the solvent in which this adhesive agent is soluble is phenols mainly composed of cresol, or strong acids such as formic acid, concentrated sulfuric acid and concentrated hydrochloric acid. Thus, there are problems in safety and workability.
Also the method of giving the adhesion effect to polypropylene and polyethylene by controlling a composition ratio of ethylene/propylene using an ethylene-propylene copolymer is generally available. However, it is difficult to balance the adhesiveness to both substrates, and the tack is likely generated.
This way, various adhesive agents and adhesion technologies for polyethylene have been proposed, but nothing having the effect on all of low density polyethylene, linear low density polyethylene and high density polyethylene which are different in physical property and having the adhesiveness to the other polyolefin substrates and the polar substrates simultaneously is available at all.
Accordingly, an adhesive agent exhaustively having the adhesiveness to the polyolefin substrates including the above various polyethylene and the various polar substrates, having a good coating property and capable of adhering at low temperature has been required.